1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and a process for production of the positive electrode material thereof, said nonaqueous electrolyte secondary battery having a positive electrode made of a carbonaceous material, a negative electrode made of metallic lithium or a material capable of occluding and releasing lithium, and an electrolyte containing a lithium salt as the solute.
2. Background Art
There have been a variety of nonaqueous electrolyte secondary batteries which find use in various fields owing to their high energy density acquired after charging. Unfortunately, they suffer the disadvantage of becoming nearly or completely unusable after a certain number of repeated charging-discharging cycles. In order to improve the cycle life of the secondary battery of this kind, the present inventors carried out a series of researches, with their attention paid to a nonaqueous electrolyte secondary battery which has a positive electrode formed from a graphitized carbonaceous material, an electrolyte containing a lithium salt, and a negative electrode formed from metallic lithium or a material capable of occluding and releasing lithium.
There has long been known a nonaqueous electrolyte secondary battery which has a positive electrode formed from a graphitized carbonaceous material, an electrolyte containing a lithium salt, and a negative electrode formed from metallic lithium. Also, attempts have been made to improve its cycle characteristic by forming its negative electrode from a carbonaceous material capable of occluding and releasing lithium (as disclosed in Japanese Patent Application Laid-open Publication Nos. Sho61-7567 and Hei2-82466, for example). These attempts were made in view of the fact that metallic lithium undergoes dissolution and deposition repeatedly after charging-discharging cycles, thereby forming dendrites and causing passivation, which reduces the cycle life.
The nonaqueous electrolyte secondary battery constructed as mentioned above is usually assembled in its discharged state, so that it remains incapable of discharging until it is charged. The charging-discharging reaction will be explained below with reference to a battery which has a negative electrode formed from a graphitic material capable of reversibly occluding and releasing lithium.
When the battery is charged for the first cycle, the following reactions take place. Anions in the electrolyte are occluded into the positive electrode (or graphitic material), and cations (or lithium ions) in the electrolyte are occluded (by intercalation) into the negative electrode. In the positive electrode are formed a graphite intercalation compound of acceptor type, and in the negative electrode is formed a graphite intercalation compound of donor type. During discharging that follows charging, the cations and anions which have been occluded respectively into the two electrodes are released (by deintercalation), and the battery decreases in voltage. This charging-discharging reaction may be represented by the following equations.
Positive electrode:(discharging) Cx+A−=CxA+e−(charging)
Negative electrode;(discharging) Cy+Li+e−LiCy (charging)
In the secondary battery of this kind, the positive electrode utilizes the reaction which reversibly forms a graphite intercalation compound containing anions as the result of charging and discharging.
A variety of positive electrode materials have so far been considered as listed below: Graphitized carbon fiber (Japanese Patent Laid-open Publication No. Sho61-10882), exfoliated graphite sheet (Japanese Patent Application Laid-open Publication No. Sho63-194319), woven cloth of graphitized carbon fiber (Japanese Patent Application Laid-open Publication No. Hei4-366554), plastic-reinforced graphite, natural graphite powder, pyrolized graphite, graphitized carbon fiber grown from gas phase, and PAN-derived carbon fiber.
Unfortunately, the battery of this kind suffers the disadvantage of decreasing in discharge capacity after repeated charging-discharging cycles. This results mainly from the deterioration of the positive electrode material, which takes place as follows. As the charging-discharging cycles are repeated, anions of comparatively large molecules are repeatedly occluded into and released from the graphitic material. The repeated occlusion and release break graphite crystals and crack graphite particles, making part of graphite incapable of charging and discharging any longer. Decrease in discharging capacity accelerates particularly in the case the charging-discharging cycles are repeated, with the charging capacity held above a certain level (about 24 mAh/g). In this case, too, the electrode can hardly keep its shape.
On the other hand, it has been confirmed that the battery having a positive electrode of graphitized carbon fiber grown from gas phase exhibits an extended life longer than 400 cycles if it is charged and discharged, with the charging capacity limited to 36 C/g (=10 mAh/g) per unit weight of graphitic material. However, the problem with low capacity still remains unsolved.
Incidentally, the term “graphitizing” used in this specification means the solid-phase transition of amorphous carbon into graphite by thermal energy. To be more specific, it implies heat treatment at 2000° C. or above irrespective of the degree of crystallinity after graphitization. The term “carbonaceous material” denotes any substance (including organic polymeric compounds) composed mainly of carbon atoms. It is not specified by the regularity of atom arrangement. Likewise, the term “graphitic material” denotes a solid substance composed mainly of carbon atoms forming the crystalline structure with three-dimensionally regular arrangement. It may or may not be the graphitized material mentioned above. Also, the graphitic material is included in the carbonaceous material.
The present invention was completed to tackle the above-mentioned problems. It is the principal object of the present invention to provide a nonaqueous electrolyte secondary battery having a large capacity and an outstanding cycle characteristic and to provide a method for production of the positive electrode material of the secondary battery.